macOS onboarding requires GitHub sign-in and hides the BYOK-only path

### Short summary

The macOS first-run onboarding requires GitHub authentication and does not expose the intended option to start using BYOK without signing in.

### Affected version or release

GitHub Copilot app 1.1.11

### Installation context

Local desktop installation on macOS 26.6.1 (Build 25G76); first-run onboarding before any organization or repository is selected.

### What happened?

When launching the GitHub Copilot app while signed out, the first screen presents only these actions:

- Sign in to GitHub
- Sign in to GitHub Enterprise Cloud (*.ghe.com)

There is no visible way to skip authentication, continue to BYOK setup, or use a local/external model provider without first signing in to GitHub.

This conflicts with the intended onboarding behaviour described by Pierce Boggan: users should be able to start using BYOK without logging in.

### Steps to reproduce

1. Install or open GitHub Copilot app 1.1.11 on macOS.
2. Ensure there is no authenticated GitHub session in the app.
3. Launch the app.
4. Observe the first onboarding screen.

### Expected behavior

The onboarding flow should provide a visible option to skip GitHub authentication and proceed directly to BYOK/provider configuration. And use the GitHub Copilot app without having a github account. For non-github repositories

### Additional context

Related conversation:

- [Reporter's original screenshot post](https://x.com/jitendravyas/status/2090522508596576311)
- [Expected BYOK-without-login behaviour](https://x.com/pierceboggan/status/2090520156661579959)
- [Request to log this bug](https://x.com/pierceboggan/status/2090561921812689073)

Reproduce the signed-out macOS first-run onboarding described in the issue and trace the entry point that presents only GitHub sign-in options; no source file or test is named. Done means a user can skip authentication, reach BYOK/provider configuration, and use the app for non-GitHub repositories without a GitHub account.

索引モデルが issue の本文から書いたものです。